A(V/V) = K x

x

RF

RG

1

1 + e

N - VG

VC

LMH6505

SNOSAT4E – DECEMBER 2005 – REVISED APRIL 2013

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As the IRG_MAX limit is approached with increasing the input voltage or with the lowering of RG, the device's

harmonic distortion will increase. Changes in RF will have a dramatic effect on the small signal bandwidth. The

output amplifier of the LMH6505 is a current feedback amplifier (CFA) and its bandwidth is determined by RF. As

with any CFA, doubling the feedback resistor will roughly cut the bandwidth of the device in half.

For more about CFAs, see the basic tutorial, OA-20, Current Feedback Myths Debunked, (literature number

SNOA376), or a more rigorous analysis, OA-13, Current Feedback Amplifier Loop Gain Analysis and

Performance Enhancements, (literature number SNOA366).

OTHER CONFIGURATIONS

1. Single Supply Operation

The LMH6505 can be configured for use in a single supply environment. Doing so requires the following:

(a) Bias pin 4 and RG to a “virtual half supply” somewhere close to the middle of V

+ and V− range. The other

end of RG is tied to pin 3. The “virtual half supply” needs to be capable of sinking and sourcing the

expected current flow through RG.

(b) Ensure that VG can be adjusted from 0V to 2V above the “virtual half supply”.

(c) Bias the input (pin 2) to make sure that it stays within the range of 2V above V− to 2V below V+. See the

Input Voltage Range specification in the Electrical Characteristics table. This can be accomplished by

either DC biasing the input and AC coupling the input signal, or alternatively, by direct coupling if the

output of the driving stage is also biased to half supply.

Arranged this way, the LMH6505 will respond to the current flowing through RG. The gain control relationship

will be similar to the split supply arrangement with VG measured with reference to pin 4. Keep in mind that

the circuit described above will also center the output voltage to the “virtual half supply voltage.”

2. Arbitrarily Referenced Input Signal

Having a wide input voltage range on the input (pin 2) (±3V typical), the LMH6505 can be configured to

control the gain on signals which are not referenced to ground (e.g. Half Supply biased circuits). This node

will be called the “reference node”. In such cases, the other end of RG which is the side not tied to pin 3 can

be tied to this reference node so that RG will “look at” the difference between the signal and this reference

only. Keep in mind that the reference node needs to source and sink the current flowing through RG.

GAIN ACCURACY

Gain accuracy is defined as the actual gain compared against the theoretical gain at a certain VG, the results of

which are expressed in dB. (See Figure 42).

Theoretical gain is given by:

where

•

K = 0.940 (nominal) N = 1.01V

•

VC = 79 mV at room temperature

(3)

For a VG range, the value specified in the tables represents the worst case accuracy over the entire range. The

"Typical" value would be the difference between the "Typical Gain" and the "Theoretical Gain." The "Max" value

would be the worst case difference between the actual gain and the "Theoretical Gain" for the entire population.

GAIN MATCHING

As Figure 42 shows, gain matching is the limit on gain variation at a certain VG, expressed in dB, and is specified

as "±Max" only. There is no "Typical." For a VG range, the value specified represents the worst case matching

over the entire range. The "Max" value would be the worst case difference between the actual gain and the

typical gain for the entire population.

14

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